A neutron traveling with a velocity ( v ) and kinetic energy ( E ) collides perfectly elastically head-on with the nucleus of an atom of mass number ( A ) at rest. The fraction of total energy retained by the neutron is:
a) (A/2A+1)
b) (2A/2A+1)
c) (A+1/2A+1)
d) (2A+1/A)